linux-kernel - Re: [PATCH 2/7] drivers/hwmon: add local variable for newly allocated attribute

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Message-ID: <2023100908-pouring-synapse-75bd@gregkh>
Date:   Mon, 9 Oct 2023 19:27:51 +0200
From:   Greg KH <greg@...ah.com>
To:     Max Kellermann <max.kellermann@...os.com>
Cc:     Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] drivers/hwmon: add local variable for newly
 allocated attribute_group**

On Mon, Oct 09, 2023 at 06:57:35PM +0200, Max Kellermann wrote:
> This allows the compiler to keep the pointer in a register and

Maybe, maybe not, there's no guarantee for register usage.

And it doesn't matter, this is a very slow path, no registers are
required :)

> prepares for making the struct field "const".

What struct field?


> 
> Signed-off-by: Max Kellermann <max.kellermann@...os.com>
> ---
>  drivers/hwmon/hwmon.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hwmon/hwmon.c b/drivers/hwmon/hwmon.c
> index c7dd3f5b2bd5..e50ab229b27d 100644
> --- a/drivers/hwmon/hwmon.c
> +++ b/drivers/hwmon/hwmon.c
> @@ -783,6 +783,7 @@ __hwmon_device_register(struct device *dev, const char *name, void *drvdata,
>  	hdev = &hwdev->dev;
>  
>  	if (chip) {
> +		const struct attribute_group **new_groups;
>  		struct attribute **attrs;
>  		int ngroups = 2; /* terminating NULL plus &hwdev->groups */
>  
> @@ -790,8 +791,8 @@ __hwmon_device_register(struct device *dev, const char *name, void *drvdata,
>  			for (i = 0; groups[i]; i++)
>  				ngroups++;
>  
> -		hwdev->groups = kcalloc(ngroups, sizeof(*groups), GFP_KERNEL);
> -		if (!hwdev->groups) {
> +		hwdev->groups = new_groups = kcalloc(ngroups, sizeof(*new_groups), GFP_KERNEL);

So you have a const pointer AND a non-const pointer pointing to the same
thing?


> +		if (!new_groups) {
>  			err = -ENOMEM;
>  			goto free_hwmon;
>  		}
> @@ -804,14 +805,14 @@ __hwmon_device_register(struct device *dev, const char *name, void *drvdata,
>  
>  		hwdev->group.attrs = attrs;
>  		ngroups = 0;
> -		hwdev->groups[ngroups++] = &hwdev->group;
> +		new_groups[ngroups++] = &hwdev->group;

This shoul be identical, you assign both above the same way, so why
change this?

>  
>  		if (groups) {
>  			for (i = 0; groups[i]; i++)
> -				hwdev->groups[ngroups++] = groups[i];
> +				new_groups[ngroups++] = groups[i];

Same here.

thanks,

greg k-h